Conventional filter assemblies can include two or more generally disk-shaped filter elements stacked along a central hub or tube. The upper and lower surfaces of each filter element are covered with a porous filter medium, and a drainage structure that communicates with the central tube is positioned between the upper and lower filter media of each filter element. The stack of filter elements, which is arranged with a gap between each pair of adjacent filter elements, is enclosed in a housing.
Fluid is supplied to the housing, typically at high temperature and high pressure. The fluid enters the gaps between adjacent filter elements and then passes through the porous filter media covering each filter element. As the fluid passes through the filter media, undesirable substances such as particulates, aggregates, or gels are removed from the fluid. After passing through the upper and lower filter media of each filter element, the filtered fluid then flows along the drainage structure to the central tube and exits the housing via the tube.
These conventional filter assemblies are useful in a variety of industrial applications, including the filtration of fluids such as molten polymers or monomers. However, because these fluids typically have a relatively high viscosity (i.e., are very thick), a significant pressure drop can develop in the gap between the filter medium surfaces of adjacent filter elements as the fluid flows in the gap and through the media. Slight differences in the gaps between adjacent filter elements and slight differences in fluid flow rates into and within the gap can result in different pressure drops in the gaps on opposite sides of a filter element. This difference in pressure drops from one side of a filter element to the other produces a force which is exerted across the filter element and which can be large enough to cause deformation and even failure of the filter element. Consequently, spacers are frequently used in these conventional filter assemblies. The spacers are positioned in the gaps between pairs of adjacent filter elements and extend inwardly to the hub or central tube. The spacers serve to maintain equal gaps between adjacent filter elements and to prevent deformation of one filter element towards an adjacent filter element should a force develop due to slight spacing differences or slight flow rate differences.
Unfortunately, conventional filter assemblies with spacers suffer from several disadvantages. For example, because the spacers are positioned between adjacent filter elements and next to the hub, the spacers can interfere with the free flow of fluid between the filter elements. This creates stagnant areas and dead spots, which, in turn, reduce the efficiency of the filter assembly. Further, because the spacers are forced against the filter elements by the pressurized fluid and to a lesser extent by the weight of the filter elements, the spacers can compress the filter media and blind (i.e., prevent passage of fluid through) large areas of the filter media. Where the spacers are not permanently attached to the filter elements, every time the filter elements and spacers are removed, cleaned, and reassembled, an additional area of the filter media is rendered blind. This progressively reduces the efficiency of the filter assembly.
Conventional spacers have other disadvantages. For example, some spacers are disposed between filter elements with spacing members that extend radially outwardly almost to the outside diameter of the filter elements from a central ring mounted about the central hub or tube. However, the support provided by these radial members declines with increasing diameter because the distance between them increases. Another conventional spacer comprises a mesh washer with the inside and outside diameters coinciding with the outside diameter of the filter element hub and the outside diameter of the filter element, respectively. In addition to many of the disadvantages previously mentioned, these spacers have surface area which resists the flow of fluid between adjacent filter elements and increases the residence time of the fluid between filter elements.